The invention concerns a nuclear magnetic resonance (NMR) tomography apparatus with a superconducting main-field magnet made from two partial coil systems which are rotationally symmetric about a horizontal central axis z and which produce a homogeneous static magnetic field, the partial coils being mirror-symmetrically arranged with respect to a vertical transverse middle plane E.sub.q which runs at right angles to the central axis z, and with a system of grading coils, whereby at least the partial coil system of the superconducting main-field coil is accommodated within a two-part cryostat which is symmetric with respect to the transverse middle plane E.sub.q and Which comprises two slice-like partial cryostat which are connected by means of strut-like connecting elements, running largely parallel to the central axis z of length g, into a complete cryostat and which exhibits a cylindrical room temperature bore having a diameter d.sub.B and running parallel to the central axis z.
An NMR tomography apparatus of this kind is, for example, known in the art from DE 39 07 927 A1.
Although, in the past, tomography systems have been practically exclusively used for diagnosis, the future will see an ever increasing need for combined systems with which therapeutic measures can be directly followed and checked using tomography apparatuses. A large number of therapeutic measures, for example, invasive surgery, in particular micro-surgery or radiation treatments necessitate as free an access to the patient as possible. This is, however, hindered in conventional NMR systems by all three field-producing components, namely the main-field magnet, the gradient magnet system, as well as the RF resonator.
The problem with respect to the main-field magnet has already been solved with the magnet system which has become known through DE 39 07 927 A1 comprising a transverse field magnet with a Helmholtz-like coil which, because of its construction, only slightly limits the free sideward access to the measurement volume.
The problem of as uninhibited a side access through the RF resonator as possible has been solved by the subsequent laid-open application DE 42 30 140 A1 of the same applicant having the title "YIN-YANG-RF-coil" with the assistance of the asymmetric saddle coils described therein, whereby the important tomographic property of a sufficiently high homogeneity for the produced RF field is fulfilled.
In order to facilitate the carrying out of minimally invasive procedures (so-called keyhole surgery) with which, due to the lack of a direct field of view onto the operation region caused by the, under certain circumstances, extremely small operation opening in the patient, NMR monitoring provides the operator with an important assistance for the on-line observation of the operation, the transverse access to the measurement volume within the NMR apparatus has as large a sideward access angle as possible, and should not be limited by the gradient coil system. This problem has been solved by the subsequently published DE 42 30 145 A1 "Gradient coils for Therapy Tomographs" which also stems from the same applicant.
In the above-mentioned superconducting main-field magnet of DE 39 07 927 A1 having good transverse access possibilities, the axially outer partial coils exhibit an outer diameter of approximately 2 m. It is possible to produce homogeneous magnetic fields in a spherically-shaped investigational volume having a diameter of about 0.4 m using such a magnet system with a maximum relative field strength fluctuation of 10 ppm. If magnet coils are cooled with liquid helium and comprise commercial superconducting wire based on niobium titanium alloys, it is possible to produce field strengths of 0.5 T and above. Due to the Lorentz forces, the partial coils lying on both sides of the transverse access opening pull towards each other with a total force of over 300,000N. This force must be captured by support construction towards which end strut-like connecting elements are provided for in the known main-field magnet system which connect the two slice-like partial cryostats and which facilitate an exchange of cryogenic liquids between these parts.
These connecting elements of the outer container of the cryostat or the parts enclosing same necessarily pass through the transverse access region and, in principle, limit its unrestricted use.
The horizontal configuration of the magnet system facilitates the bringing of a patient on a patient bed into the axial bore of the system, whereby the transverse access opening allows, in principle, for the possibility of carrying out therapeutic measures on the patient and of monitoring their progress with NMR investigations. A particularly large degree of freedom in carrying out such therapeutic measures would allow, if possible, for a first person (for example a therapist) to stand erect during the NMR investigation in the vicinity of the transverse access opening next to the patient and, if necessary, to bend down over the patient. In addition it would be quite helpful if, in addition, a second person (for example an assistant) could likewise stand erect on the other side next to the patient and, for example, provide manual assistance. The possibilities towards this end are, however, in principle rendered more difficult by the fact that the partial coils located on both sides of the transverse access openings attract each other with a large force and must necessarily be supported with respect to each other.
Towards this end it is necessary that the support body, which supports all partial coils of the main-field magnet be a single integrated mechanical unit which is embodied in the vicinity of the transverse access openings by the strut-like connecting elements. In order for the construction to be mechanically secure, these connecting elements should be loaded, when accepting the Lorentz forces, solely under compression and in no event under tension. It is, however, thereby necessary for these connecting elements as well as the essentially tube-shaped radiation shields and pressure-vessel portions of the cryostat which surround them to be located in the vicinity of the transverse openings and to thereby limit the freedom of motion of the therapist or therapists.
In contrast thereto it is the purpose of the present invention to present an NMR tomography apparatus of the above-mentioned kind wherein, on the one hand, the cryostat for the superconducting main-field magnet is constructed in a mechanically stable fashion and wherein, with the occurrence of Lorentz forces, its individual components are solely subjected to compressional loads, whereby, however, the construction facilitates an optimal transverse access and free space for the activities of, for example, a therapist or an assistant working on a patient lying in the investigational volume of the tomography apparatus.